It is desirable that epoxy resins which are intended to be used as a resin sea or adhesive in a high-voltage electronic part have reduced internal stress while maintaining their intrinsic heat resistance. In order to reduce the internal stress of epoxy resins while maintaining the heat resistance, various methods for dispersing a silicone oil or silicone rubber in an epoxy resin to thereby make a dispersion which has a sea-island structure have heretofore been proposed.
Many of the known s are directed to the use of a solid epoxy resin composition as sealants for semiconductors. However, no example is known which is directed to the use of liquid epoxy resins. Silicone easily separates from liquid resins before and during curing because of its properties and reactivity, with the result that the silicone oil often bleeds out of the surface of the resin or is en completely separated from the resin to form a distinct layer on its surface. In order to solve these problems, it has heretofore been proposed to employ one or both of the following measures:
(1) Addition of a dispersing agent which is compatible with epoxy resins and silicone compounds.
(2) Curing of silicone compounds that form the islands.
For example, Japanese Pal Lent Laid-Open Nos. 3-167250 and 1-268717 disclose the combination of the above-mentioned measures (1) and (2), and Japanese Patent Laid-Open No. 62-158716 discloses the above-mentioned measure (1).
However, even employing the above-mentioned measure(s) (1) and/or (2), there still remain problems to be solved. In more concrete terms even when employing measure (1), some dispersing agent: themselves have low reactivity with epoxy resins. In such cases, the dispersing agent which is added bleeds out during the curing of the epoxy resin. On the other hand, the curing of a silicone compound that form islands according to the measure (2) results in an increase in the viscosity of the epoxy resin composition which increases the difficulty in handling the composition.
In addition, it is necessary to heat the whole epoxy resin composition during the reaction of the silicone compound therein, and therefore an increase in the cost of the resin is inevitable due to the necessary heating.